A single solution to four challenges

Number 313
September | October 2011
A single solution to four challenges
urea prilling plants
Farmers prefer pastilles to prills.
Asinglesolution
tofourchallenges
Typical problems and challenges experienced by ageing urea prilling plants include high
ammonia and dust emissions, low prill quality, competition from granules in the market and
demand from farmers for additional nutrients for their crops. In this article, Mark Brouwer
of UreaKnowHow.com explains how the Rotoform pastillation system from Sandvik Process
Systems meets these challenges and outlines a number of additional benefits.
M
ost of the urea plants around the
world operate a prilling tower; the
majority of these plants are now
several decades old and their capacities
have been gradually pushed to about 120
to 130% of their original design capacity.
These plants typically face several problems
or challenges: high ammonia and dust emissions from the prilling tower, low prill quality,
competition from granules in the market and
demand from farmers for additional nutrients for their crops. The solution to all four
challenges can be provided by Sandvik’s
Rotoform pastillation system.
High emissions
In prilling towers the urea melt falls down
inside a concrete structure, cooling and
crystallising against a large quantity of
upward-moving air. In some prilling towers
this air flow is created using fans; in others
natural draft is used. However while the
draft is created, the air becomes loaded
with urea dust particles and ammonia, and
is typically emitted into the atmosphere
from the top of the prilling tower.
In some cases a dust scrubber is
installed on top of or alongside the prilling
Nitrogen+Syngas 313 | September - October 2011
tower to reduce the urea dust emissions. In
a limited number of cases, the dust scrubber
is combined with an acid washer to reduce
the ammonia emissions. These scrubber
systems are only feasible in so-called forced
draft prilling towers, where air fans are available. Scrubber systems are not feasible in
natural draft prilling towers as the pressure
drop over the scrubbers is too high, unless
extra air fans are installed.
The amount of urea dust and ammonia emissions are directly related to the
load on the prilling tower (there is a linear
relationship)1.The relative emission figures
3
urea prilling plants
Fig 1: Sandvik Rotoform pastillation process for urea
can be influenced by the air flow, but one
should realise it is not a solution but a dilution of the problem.
Low prill quality
Increasing the load on a prilling tower can
have negative consequences for the prill
quality. Higher moisture contents and
higher temperatures cause more dust formation and an increased likelihood of caking problems2.
Caking can be so severe that the product becomes damaged when it is loaded
on trucks or cars. Part of the product will
be lost and complaints from customers
may follow. Measures need to be taken.
Competition from granules
The quality of prills is significantly lower
than that of granules, the main difference being the lower strength and smaller
size of prills. Research has shown that
once a farmer has used granules, he will
not choose prills again, unless the price
is lower. Most new urea plants are large
scale granulation plants and are located
in low feedstock areas; these plants are
export driven and compete with prills.
From bulk to specialty products
Urea prilling plants that are several dec-
4
ades old and located inland will typically
have built up a strong relationship with its
customers. When competitors with very
low feedstock costs offer their products
for a lower price, it can be very difficult to
compete on price if your plant capacity is
lower and your feedstock costs are higher.
However, a strong customer relationship does provide a competitive advantage. More and more farmers are starting
to realise that their crop needs more nutrients than just nitrogen (N). Other primary
plant macronutrients include phosphates
(P) and potassium (K); others – carbon (C),
oxygen (O) and hydrogen (H) – are available through air (via the photosynthesis
process) and water. Then there are the
secondary macronutrients: calcium (Ca),
magnesium (Mg) and sulphur (S) and the
micronutrients boron (B), copper (Cu),
iron (Fe), manganese (Mn), zinc (Zn),
chloride(Cl), nickel (Ni), and molybdenum
(Mo), which are also essential for plant
growth but required in much smaller quantities. Finally, silicon (Si) and cobalt (Co)
are also beneficial for some plants in even
smaller quantities. A deficiency of any single nutrient is enough to limit growth.
In general, nitrogen application efficiencies are very low as between 30 and
50% of the nitrogen supplied to the soil
is typically lost to air and water, causing
more and more environmental problems.
In Canada, for example, urea has become
the broadcast fertilizer of choice for many
winter wheat growers on the Canadian Prairies, especially since ammonium nitrate
fertilizer was removed from the market.
However, the problem with surface applications of urea is that the N is susceptible to
ammonia volatilisation. The urea molecule,
in the presence of moisture and the soil
enzyme urease, converts to ammonium
carbonate, which can lead to the production of ammonia gas.
Adding slow release substances to urea
prills contributes to higher efficiencies,
resulting in less environmental problems
and less fertilizer product lost and wasted.
Producing specialty fertilizer products
based on urea therefore fulfils farmers
needs; these are considered to be higher
value products attracting a higher price.
Having a close relationship with your customers gives you the competitive advantage and opportunity to produce and sell
higher margin specialty products.
The solution
The solution to all four of these challenges
can be met by Sandvik’s Rotoform pastillation system.
Figure 1 shows a simplified flow
scheme of the Rotoform system.
The feed to the Rotoform unit is urea
melt with a concentration of 99.6 wt-% and
in existing urea plants can be branched off
Nitrogen+Syngas 313 | September - October 2011
urea prilling plants
Fig 2: Rotating shell delivers droplets of the required size
Urea pastilles on the steel cooling belt
Below left: Rotoform skid mounting equipment
showing the hood which captures and ammonia and
dust emissions. Right: Front view of installation of
urea plant with four Rotoform units.
from the urea evaporation section downstream of the urea melt pumps.
Urea is introduced under pressure (2-3
barg) in molten form to the drop former.
The Rotoform HS (High Speed) drop
former, patented by Sandvik, consists of
a heated stator and a perforated rotating
shell that turns concentrically around the
stator to deposit drops of urea across the
full width of the belt. The circumferential
speed of the Rotoform unit is synchronised
with the speed of the steel belt cooler
ensuring that the drops are deposited on
the belt without deformation and, after
solidification, results in regular pastilles
with an optimum shape.
The rotating shell (Fig.2) contains
rows of small holes which are sized to
deliver the required product size. The heat
released during crystallisation and cooling
is transferred by the stainless steel belt
to the cooling water. The cooling water
is sprayed against the belt underside,
absorbs the heat and is collected in pans,
cooled in a cooling system (cooling tower)
and returned to the Rotoform units.
Under no circumstances can the cooling water come into contact with the urea
product.
The use of formaldehyde is not neces-
Nitrogen+Syngas 313 | September - October 2011
sary in this technology to realise pastilles
with a high crushing strength. The pastilles
are very uniform.
After solidification the pastilles are
smoothly released from the steel belt via
an oscillating scraper. The product then
falls directly onto a conveyor belt for transfer to storage. The section above the moving steel belt is enclosed with a hood and
vented to an existing vent system.
There are no large air flows involved in
this technology and there is no visible urea
dust emission. The ammonia vapours that
are produced can easily be captured in a
simple atmospheric absorber; this results
5
urea prilling plants
in negligible emissions of ammonia and
urea, a unique feature of this technology.
By installing one or more Rotoform lines
in parallel with a prilling tower, the load on
the prilling tower will be reduced, leading to
lower ammonia and urea dust emissions
from the prilling tower and a higher prill
quality. For example, reducing the load on
the prilling tower from 1,300 to 1,000 t/d
by installing two Rotoform lines in parallel
will reduce the ammonia and dust emission
figures by approximately 23% (300/1300).
Let’s show an imaginative but realistic
example how the switch from prills can
take place.
Figure 3 shows the production capacity of a typical urea plant over time. The
plant has a design capacity of 1,000 t/d
prills and during its first years will be able
to increase its actual production to some
1,300 t/d prills, typically by making use
of the design margins and by creep debottlenecking technologies. Dust and ammonia emissions from the prilling tower will
increase in line with the increase in production as these are linear related.
So, as plant capacity increases from
1,000 t/d to 1,300 t/d, dust emissions will
increase from 100 mg/Nm3 to 130 mg/Nm3
and ammonia emissions from 80 mg/Nm3
to 104 mg/Nm3, as can be seen in Fig.4.
When pastilles are produced the load
on the prilling tower will be reduced, as will
the dust and ammonia emission figures.
In this way one can reach emission figures
which are acceptable to local governmental regulations.
Furthermore, while the load on the
prilling tower decreases, the prill quality
increases. For example, Figure 5 shows
that after start up of the plant the prill quality was considered good. After increasing
the load on the plant and prilling tower
the prill quality became problematic. Too
much dust, high moisture content, high
prill temperatures, low strength and caking troubled the production and sales of
the prills. As pastille production reduced
the load on the prill tower, the prill quality
became good and even excellent again.
Finally, if you operate a forced draft
prilling tower, the power consumption per
tonne of urea will reduce by about 9 kWh/t
as the Rotoform unit consumes less power
than a forced draft prilling tower.
The quality of the pastilles is better than the quality of prills. It is similar
and in some aspects even better than
the quality of granules. Table 1 provides
an overview of the product properties of
6
Table 1: Typical properties prills, granules and pastilles
Average diameter, mm
Moisture, wt-%
Formaldehyde content, wt-%
Shape
Crushing strength, N
Product temperature, °C
Prills
Granules
Pastilles
1.5-1.9
0.15-0.30
0.1-0.3
(in case of export)
Spherical
12 (1.7 mm)
60-80
2-4
0.10
0.3-0.55
2-5
0.10
Only when
required
Split-pea
40-75 (3 mm)
40-45
Spherical
40 (3 mm)
50-60
Fig 3: Production capacity of prills and pastilles in t/d vs age of the plant in years
1,400
1,200
1,000
800
600
400
200
0
1
3
5
7
9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39
prills, t/d
pastilles, t/d
Fig 4: Dust and NH3 emission from prilling in mg/Nm3 vs age of the plant in years
140
120
100
80
60
40
20
0
1
3
5
7
9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39
dust emission, mg/Nm3
NH3 emission, mg/Nm3
Fig 5: Prill quality vs. age of the plant in years
Excellent
Good
Problematic
1
3
5
7
9
11 13 15 17 19 21 23 25 27 29 31 33 35 37 39
Nitrogen+Syngas 313 | September - October 2011
urea prilling plants
Fig 6: Flow scheme to produce urea + AS
Blending unit to produce urea +AS
Urea
Urea + AS 20%
Urea + AS 40%
Urea + KCl 20%
Urea + KCl 43%
Urea + sulphur
AN + AS 15%
NPK
Sulphur bentonite 10%
prills, granules and pastilles. For example,
the strength is similar to the strength of
granules even without any formaldehyde.
The size of the Rotoform product is the
same as for granules, moreover the size
can be easily varied between 1 and 5 mm.
The pastilles are extremely uniform, even
more than granules. It goes without saying
that the Rotoform product quality is significantly better than prills. Farmers who have
applied the pastilles have been very satisfied and were prepared to pay a premium
price above prills.
From Table 1 one can conclude that
granules and pastilles have some significant advantages over prills: Granules
and pastilles can be produced with larger
average diameter (which can be specified
over a relatively wide range) and are significantly stronger than prills.
A Rotoform system with a typical capacity of 130-140 t/d can be used to produce
all kinds of specialty urea products in addition to fertilizer grade urea. The following
products have been already produced with
a Rotoform pastillation system:
l fertilizer grade urea
l technical urea for urea-formaldehyde,
melamine, ad blue production
l urea blended with macronutrients
l urea blended with micronutrients
l urea with ammonium sulphate
l ammonium nitrate
l ammonium nitrate derivatives (KCl,
Zeolite, AS < 10% and NPK)
l NPK complex fertilizer from nitrate and
urea route
l calcium nitrate
l magnesium nitrate
l sulphur bentonite
The production processes for these
specialty urea products is fairly simple and
straightforward. In some cases a liquid
additive can be simply injected into the
urea melt, in other cases a blending unit
is applied to grind and mix the solid additives in the urea melt (Fig.6).
For example, for urea plus ammonium
sulphate (AS), urea melt and solid AS are
mixed and grinded in two steps in a blend-
ing unit, which is a standard packaged
unit. Up to 40 wt-% AS has been mixed with
urea with excellent results.
The introduction of specialty urea products can be realised smoothly by starting off with a single Rotoform line, which
can be brought into and out of production
whenever required, and then by adding
additional lines in parallel when market
demand increases. The pictures above
show various specialty urea products. nх
Nitrogen+Syngas 313 | September - October 2011
References
1. 2010 02 Brouwer UreaKnowHow.com Dust
Emission of Prilling Towers.
2. 2010 01 Brouwer UreaKnowHow.com Forms
of Urea & Background of Caking.
3. 2010 03 Baeder SPS UreaKnowHow.com
Rotoformer Pastilles the Sustainable Premium product.
4. 2010 04 Baeder SPS UreaKnowHow.com
Rotoformer applications.
5. IFA website.
6. Nitrogen & Syngas 2011 Presentation,
Kumar Swamy, Sandvik Process Systems.
7
Upgrade granulation
of urea, sulphur bentonite
and other fertilizers
Urea
Urea + sulphur
Urea + AS
Sulphur bentonite
Are you interested in granulation of urea, urea &
mixtures, ammonium nitrate and mixtures and
sulphur bentonite?
Take advantage of Sandvik Process Systems’ proven
Rotoform granulation system and start producing highquality granules in a more efficient, cost effective and
environmentally-friendly manner.
The process, already the world’s most widely used system for
sulphur solidification, is ideal for debottlenecking or
revamping urea and other fertilizer processing facilities,
enabling fast, controlled production with no need for
recycling, scrubbing or crushing.
Highly efficient process
• From liquid to solid in a single step
• High quality, virtually dust-free pastilles
• Suitable for urea and nitrate fertilizers, technical urea,
AdBlue, animal feed, speciality urea products and more.
Environmentally-friendly production
• Indirect heat transfer – no contact between product and
cooling media
• No separate air cleaning necessary; low dust/ammonia
emissions
• Low sound emissions
Significant commercial advantages
• Initial investment costs significantly lower than
competitive systems
• Very low energy consumption: 4kW/t for urea granulation
• No use of chemicals such as formaldehyde.
Sandvik Process Systems
P. O. Box 42 62 • D-70719 Fellbach/Germany
Tel: +49 (0) 711-51 05-0 • Fax: +49 (0) 711-51 05-196 • www.processsystems.sandvik.com
E-mail: [email protected]